In a wide variety of environmental, occupational, military and industrial scenarios, fine particles, typically within the size range from a few tenths of a micrometer to a few hundred micrometers, play an important role. Environmental airborne particles, usually comprising mineral dusts, combustion products and biological particles, which are carried by winds and other air movement, can result in breathing difficulties, allergic reactions a possible degradation of the body's immune system. Occupational particles can contaminate industrial products and processes and can also present a respirable health hazard, such as in the case of asbestos fibres or fugitive pharmaceutical powder particles. In the military field, the deliberate generation of hazardous aerosols has posed a major threat since their first substantial use in World War I, and today a wide variety of biological and chemical weapons are believed to be possessed by both national governments and terrorist organisations.
The in-situ characterization of airborne particles has therefore become an important objective in both civilian and military fields, and considerable effort has gone into developing techniques which can analyze certain particle parameters and provide some degree of identification or classification. Moreover, since even brief exposure to some of the aforementioned aerosols can damage health and may even prove fatal, the speed of response of the measurement technique has been an important consideration.
A potentially powerful technique of airborne particle analysis involves the introduction of individual particles into a near vacuum where they are fragmented using an intense laser light pulse. The resulting atomic and molecular fragments are then measured using a time-of-flight mass spectrometer or similar, yielding a detailed assessment of the material content of the particle. (See for example, Marijnissen J et al, “Proposed on-line aerosol analysis combining size determination, laser induced fragmentation, and time-of-flight mass specrometry”, Journal of Aerosol Science, volume 19, pages 1307-1310, 1988). Such methods offer a high degree of particle discrimination but remain expensive and cumbersome to implement and, because they are comparatively slow in terms of the rate at which individual particles can be analyzed, they do not offer the real-time aerosol analysis capability (ie: response to a change in aerosol composition within a few seconds) desired in monitoring applications.
Of other possible particle characterization techniques, those based on elastic optical scattering have become popular because they offer genuine real-time non-destructive particle analysis. Here, the term elastic denotes that the scattered light is at the same wavelength as the illuminating light. In their simplest form, optical scattering instruments are designed to draw ambient airborne particles through a measurement space. A light source, usually a laser, illuminates the measurement space and the particles scatter some radiation to an appropriately positioned detector. The magnitude of the scattered radiation may, to a first order, be used to determine a particle sizes and number illuminated at any instant. Whilst comparatively straightforward to implement, simple light scattering techniques such as these do not yield sufficient information about the particles to provide anything other than a very superficial overview of the ambient aerosol. They do not, for example, provide any indication of the material nature of the particles; whether the particles are of solid or liquid form; or whether the particles are of biological or non-biological origin.
In order to discriminate more effectively between airborne particles of different types, a number of methods have been developed which measure multiple parameters from individual particles in addition to their (optical scattering) size. For example, analysis of the spatial distribution of light scattered by individual airborne particles passing through the measurement space of an optical scattering instrument has proved to be an effective method of improving particle discrimination. This is because the spatial pattern or scattered light contains information relating to the shape of the scattering particle. Examples of instrument geometries which embody this approach to spatial scattering analysis are described in: ‘Portable Particle Analysers’, Ludlow, I. K. and Kaye P H. European Patent EP 0 316 172, Jul. 1992; ‘Particle Asymmetry Analyser’, Ludlow, I. K. and Kaye, P. H. European Patent EP 0 316 171, Sept. 1992.; ‘Apparatus and Method for the Analysis of Particle Characteristics using Monotonically Scattered Light’, Kaye, P.H. and Hirst, E. U.S. Pat. No. 5,471,299, Nov. 28, 1995; and ‘Hazardous Airborne Fibre Detector’. Hirst E. and Kaye, P.H. UK Patent Application No: 9619242.2; filed 14th Sep. 1996. These may be considered as prior art.
However, light scattering analysis instruments of the type described above cannot discriminate particles on the basis of their material structure. For example, a non-biological silicate-based particle may yield an essentially identical spatial scattering pattern to a biological cell of similar size and shape. In order to discriminate particles on the basis of their material structure it is necessary to employ other techniques such as an analysis of light which is scattered inelastically by the particle. Such light is manifest as either a fluorescence emission or, far more weakly, a Raman emission. Since useful Raman signals from individual microscopic particles in flow have, to date, proved unattainable, they will not be discussed further here. In contrast, several workers have demonstrated successful measurement of fluorescent spectra from single airborne particles and have used this technique to attempt particle discrimination on the basis of fluorescence. For example, Pinnick et al (‘Fluorescent Particle Counter for Detecting Airborne Bacteria and Other Biological Particles’ Pinnick R G et al., Aerosol Science and Technology, volume 23, pages 653-664, 1995) developed an instrument in which a stream of airborne particles passes through a measurement space and is illuminated with light at 488 nm wavelength from an Argon-ion laser. The light excites some naturally occurring fluorophores within the particles and the fluorescence emission spectrum between 500 nm and 800 nm wavelength is recorded and analysed. Based on the fact that biological particles such as spores produced measurable fluorescence, the authors proposed the technique as a possible means of discriminating biological from other non-biological particles that may be present in an environment. Other workers, (for example see Hairston P P et al, “Design of an instrument for real-time detection of bioaerosols using simultaneous measurement of particle aerodynamic size and intrinsic fluorescence”, Journal of Aerosol Science, vol. 28, no. 3, pages 471-482, 1997), have combined a measurement of the magnitude of fluorescence from a particle with a measure of its size, in this case the aerodynamic size of the particle. This dual-parameter measurement approach provides a greater degree of particle discrimination than measurement of particle fluorescence alone. This method has been extended by Kaye P H et al. “Simultaneous light scattering and intrinsic fluorescence measurement for the classification of airborne particles U”, Applied Optics, volume 39, number 21, pp 3738-3745, to incorporate a method of determining the shape of individual particles from an analysis to the spatial distribution of light scattered by the particle. These methods too may be considered prior art.
However, all of the methods described above involve the analysis of individual particles at normally high processing rates. Whilst offering a high degree of particle discrimination, the methods all suffer the same problem of being expensive and complex to implement. This high cost is a result of the requirement for an intense and well-collimated light source, usually a laser, the requirement for precision optical systems, the need for complex and high-speed data processing electronics, and, normally, the need for an independent power generator to supply the instruments with electrical power over extended time periods. Because of the high cost of implementing the methods, the deployment of monitors based on them is normally limited to very small numbers of discrete monitors. In some cases, especially outdoor environments or areas of military conflict, the biological threat may appear anywhere across a large area, and the deployment of small number of discrete monitors is of limited value in rapidly detecting the threat, should it arise. What is required, therefore, is a monitor which is of sufficiently low cost and small size that it may be manufactured and deployed in very large numbers across wide areas of potential risk, or even that it could be worn or carried by every individual person in the area who may be exposed to the biological hazard. Such a monitor would ideally meet the following specification:                1. Low cost.        2. Hand portable or person wearable.        3. No reagent requirement. (i.e: no requirement for recharging chemical or biochemical assay systems).        4. Unattended operation.        5. Typically 48 to 72 hours continuous operation using built-in battery power supply or similar.        6. Maximum response time of typically 10s. i.e: will detect the presence of biological particles in an environment within a time period short enough to prevent significant exposure of individuals to the hazard.        
A detector which meets this specification is described in the present applicant's earlier UK patent application 0210116.0 ‘Detector for airborne biological particles’, which may be considered prior art.
The detector described in UK patent application 0210116.0 ‘Detector for airborne biological particles’, operates by drawing a continuous flow of ambient air through a chamber, periodically illuminating this air with ultraviolet light, and measuring the magnitude of scattered light and fluorescence emission from the particles suspended in the air. The relationship between the scatter and fluorescent signals is indicative of the presence or absence of biological particles in the air sample.
A potential drawback of this type of detector relates to the size of the volume of the illuminated air parcel (referred to as the ‘scattering volume’). With low particle number concentrations in the air, it could be possible for no particles to be present in the scattering volume and therefore for no signal to be recorded. Equally, if only a small number of particles is present in the scattering volume, the magnitude of the recorded scatter and fluorescence light signals may be too low for accurate measurement, primarily because the scatter and fluorescent light emanates from the particles in all directions and yet is collected over only a small fraction (typically ˜10% of the total).
It is an object of the present invention amongst others to overcome these limitations and offer substantial improvement in detection sensitivity over the prior art. Further objects include obtaining further economies in cost of the apparatus, with enhancements to the apparatus to provide for useful characterization of contaminant particles despite a relatively low cost construction.